1. Field of the Invention
The present invention relates to an optical device that allows an observer to observe a two-dimensional image formed by an image forming device, and to an image display apparatus equipped with the optical device.
2. Description of the Related Art
PCT Japanese Translation Patent Publication No. 8-507879 and Japanese Unexamined Patent Application Publication No. 2002-162598 disclose known examples of a virtual-image display apparatus equipped with a hologram diffraction grating. Such an apparatus allows an observer to observe a two-dimensional image formed by an image forming device as a virtual image magnified by a virtual-image optical system.
FIG. 11 schematically illustrates a virtual-image display apparatus disclosed in PCT Japanese Translation Patent Publication No. 8-507879. The virtual-image display apparatus includes an image forming device 101, a light guide plate 102, a transmissive hologram lens 103, and a transmissive hologram diffraction grating 104. Image light displayed on the image forming device 101 is emitted to the light guide plate 102, and is then collimated by the transmissive hologram lens 103 provided within the light guide plate 102. Inside the light guide plate 102, the image light is deflected by an angle for total reflection. Subsequently, the image light travels through the light guide plate 102 while undergoing total reflection therein, and then enters the transmissive hologram diffraction grating 104 where the image light is diffracted. The transmissive hologram diffraction grating 104 is provided on the same axis line as the transmissive hologram lens 103 within the light guide plate 102 and is separated from the transmissive hologram lens 103 by a predetermined distance. The diffracted image light is released from the light guide plate 102 as collimated light and is guided towards an eye of an observer.
On the other hand, FIGS. 12A and 12B schematically illustrate a virtual-image display apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-162598. This virtual-image display apparatus includes an image forming device 201, a free-form-surface prism 202, a light guide plate 203, a first holographic optical element 204, a second holographic optical element 205, a third holographic optical element 206, and a fourth holographic optical element 207. Each of the holographic optical elements will simply be referred to as HOE hereinafter. Image light displayed on the image forming device 201 travels through the free-form-surface prism 202 so as to enter the light guide plate 203. The image light incident on the light guide plate 203 is continuously diffracted and reflected by the first HOE 204, which is provided on a surface of the light guide plate 203 opposite to an incidence surface thereof, and by the second HOE 205, which is provided on the incidence surface of the light guide plate 203. In this case, the image light is set to a critical angle or more so as to undergo total reflection within the light guide plate 203. The image light with the critical angle or more travels through the light guide plate 203 while undergoing total reflection within the light guide plate 203, and is then continuously diffracted and reflected by the fourth HOE 207 and the third HOE 206 so as to be given the critical angle or less. The image light is released from the light guide plate 203 and is guided towards an eye of an observer.